In the related art, there is known an illumination device provided with a light guide plate which guides the light emitted from a light source unit and which emits the light outward (see, e.g., Japanese Unexamined Patent Application Publication No. 2004-228018A). The light guide plate includes an incidence surface on which the light emitted from the light source unit is incident. The light guide plate emits the light incident on the incidence surface to the outside from both main surfaces while guiding the light toward an emission end surface opposed to the incidence surface.
However, if the aforementioned illumination device is installed on a ceiling with the light guide plate kept orthogonal to the ceiling, the amount of light emitted from the light guide plate to be directed toward the ceiling is small. Thus, there may be a case where the ceiling around the illumination device is dark and the illuminated space is felt narrow, thereby generating an oppressive feeling.